A very common problem encountered in many industries, such as the petrochemical industry, is compensating for disturbances in the flow rate of liquid materials coming into a particular processing unit. Such disturbances are usually common and ordinary events in the routine operation of the process, for example in an olefin plant.
One of the most common and important disturbances which occurs in an olefin plant is a change in the plant feed rate. For instance, furnaces are frequently brought off-line for decoking and then brought on-line again. The plant feed disturbance caused by bringing down or starting up a furnace enters the pyrofractionator whose top products go through compression and fractionation (demethanizer, deethanizer, ethylene splitter, depropranizer, and debutanizer), creating temperature and composition control problems for the cold side fractionation of the olefin plant. Ultimately, smooth operation of a plant may be hindered. Disturbances are commonly transmitted from the point of origin through the plant by means of changes in the forward flow. The less sudden these changes, and the more the magnitude of these disturbances can be minimized, the better will be the plant controllability. Most of these processes can be adjusted to accommodate these variations with little loss in efficiency, providing the surge is not too great and sufficient time is available to adjust the process.
One strategy has been to include one or more surge tanks in the liquid flow lines or to utilize certain volume capacity ranges within existing vessels to provide temporary capacity for smoothing out the surges. The liquid levels in these vessels, e.g., surge tanks, bottoms of fractionation columns and accumulators, and so forth, may then be allowed to vary within limits so that the outlet flow changes from these vessels are significantly smaller than the instantaneous inlet flow changes. Each liquid level thus acts as a buffer for the downstream units. Thus, this surge capacity, which may be receiving flow from a number of different units, by allowing the level in the surge tank to deviate from its setpoint while staying within allowable limits, attenuates the effects of any feed flow disturbance so that the disturbances do not propagate quite as strongly and the operation of the process is much steadier.
A good surge volume control algorithm should have several important characteristics. The level in the vessel should not exceed the high and low limits to ensure that the vessel will not overflow or empty. In the absence of any disturbance over a long period of time, the level should line out at the target level. The available surge volume should be utilized effectively to minimize the effect of a feed rate change on the downstream process. The method should be relatively simple so that it can be easily maintained. Also, tuning the controller should not be difficult and should not require much effort.
This is not a new problem, and sophisticated and complicated control programs for entire refining plants exist, usually requiring a very large computer installation for implementation. Rigorous solutions of this problem, for example, typically require a quadratic programming technique. What is needed is a less complicated but robust and equally effective method and apparatus for surge volume control which can be readily and economically utilized and implemented in smaller surge control applications. Ideally, the method and apparatus could be implemented in a portable, microprocessor-based controller, thereby affording the greatest economy and versatility.